Thin film bulk acoustic wave (BAW) resonators that are used in oscillators and filters can deviate from their designed resonant frequency due to manufacturing variations in the thickness and material properties of their constituent films (also referred to as material layers or simply layers) by thousands of ppm or even more. This deviation in resonant frequency needs to be corrected to within a specified tolerance before the resonators are used in their intended applications.
The series resonant frequency (referred to as the resonant frequency and noted Fs or fs in this document) of a thin film bulk acoustic resonator (FBAR) with very thin electrodes in the thickness-extensional mode (the mode perpendicular to the resonator layer thicknesses) is given by fs=√{square root over ((c/ρ))}/(2L), where c is the stiffness, ρ is the mass density, and L is the thickness of the piezoelectric layer. In this canonical FBAR, the thickness-extensional mode acoustic waves are confined to the piezoelectric layer, so the acoustic path length is simply the thickness of the piezoelectric layer, L.
The series resonant frequency equation presented above is modified when electrodes cannot be assumed to be very thin, or if the resonator is constructed in a surface mounted resonator (SMR) or double SMR (DSMR) configuration. However, the equation still shows a strong dependence of resonant frequency on material properties (c, ρ) and layer thickness (L) for each material present in the acoustic path.
The resonant frequency of a resonator can be altered by altering its acoustic path length, either by changing the thickness of one or more layers of material present in the acoustic path, or by changing the properties of that/those material(s). Changing the thickness of a material layer is commonly referred to as changing the “mass load” of the resonator or as “mass loading” the resonator, and the material that undergoes the thickness change is commonly referred to as the “mass load” layer. An increase in the acoustic path length leads to a decrease of the resonant frequency. FIG. 1 contrasts the acoustic path lengths for three prior art devices 10, 20, and 30. Comparing device 20 to device 10, the acoustic path length of device 20 is increased over that of device 10 because mass load layer 22 is added over electrode 16. Note that mass load layer 22 can be either an insulator or a conductor. Comparing device 30 to device 10, the electrode 16 is replaced with layer 26, which can increase or decrease the acoustic path length depending on the physical properties of layers 16 and 26.
As stated above, the acoustic path length of a resonator depends on the choice of materials and thickness of layers of the resonator and the distribution of acoustic energy within the device at the resonant frequency. A frequency shift is caused by altering either the material properties or the thickness of the layer or layers. The primary acoustic path is the portion of the main body of the resonator that contains significant portions of the resonant standing wave energy, which in the case of FBARs would be the electrodes and the piezoelectric layer, as well as any temperature drift compensating layers present in the FBAR stack. In an SMR (solidly mounted resonator) or DSMR (double solidly mounted resonator), the primary acoustic path would include those portions of the Bragg reflector layers that contain significant portions of the acoustic energy of the resonator.
As stated above, one or more mass load layer can be inserted into the acoustic path of any BAW resonator in order to decrease the resonant frequency. One example of such a layer is layer 22 in FIG. 1. The secondary acoustic path is the portion of the mass load layer(s) that contains significant portions of the resonant standing wave energy. The primary acoustic path is illustrated as 48 and the secondary acoustic path is illustrated as 50 in FIG. 3. The primary and secondary acoustic paths taken together form the complete acoustic path of the resonator, which is illustrated as 12 on device 20 in FIG. 1.
Prior art methods for modifying the acoustic path length can be classified based on whether the resonator acoustic path length is modified by altering the overall physical thickness of layers making up the resonator, or by changing the material properties of layers in the acoustic path, or both.
In one known approach, extra material is added on top of the top electrode of the resonator such as layer 22 in FIG. 1, thus increasing the overall thickness and hence the acoustic path length. This material may be an extra layer of material that is added adjacent to an electrode of the resonator, or an existing layer (such as an electrode layer) is thickened. Both approaches have the effect of reducing the resonant frequency. U.S. Pat. No. 5,894,647 to Lakin et al, entitled “Method for Fabricating Piezoelectric Resonators and Product”, which issued on Apr. 20, 1999, describes a method of changing the thickness of an electrode to shift the resonant frequency. Electrodes of different thicknesses can be deposited on top of a resonator to create a range of different resonant frequencies in an array of resonators. U.S. Pat. No. 6,469,597 to Ruby et al, entitled “Method of Mass Loading of Thin Film Bulk Acoustic Resonators (FBAR) for Creating Resonators of Different Frequencies and Apparatus Embodying the Method”, issued on Oct. 22, 2002 describes a method of mass-loading the lower electrode (e.g., 18 in FIG. 1) to shift the resonant frequency. That is, by adding a layer beneath the bottom electrode of the FBAR the overall acoustic path length of the FBAR is increased.
Other methods ablate or remove material from the acoustic path to change the resonant frequency of the device. U.S. Pat. No. 5,587,620 to Ruby et al, entitled “Tunable Thin Film Acoustic Resonators and Method for Making the Same”, which issued on Dec. 24, 1996, describes a method of changing the resonant frequency by using resistive heating elements. The heating elements evaporate a tuning layer over time, until the resonant frequency has come into the desired range or value. Another method uses a laser to ablate or partially remove the electrode material, or some other mass load or sacrificial material. One exemplary patent that describes this method is U.S. Pat. No. 4,642,505 to Arvanitis, entitled “Laser Trimming Monolithic Crystal Filters to Frequency”, which issued on Mar. 5, 1984.
Other methods involve progressively altering the material properties of the mass load to change the resonant frequency. For example, U.S. Pat. No. 6,566,979 to Larson et al., entitled “Method of Providing Differential Frequency Adjusts in a Thin Film Bulk Acoustic Resonator (FBAR) Filter and Apparatus Embodying the Method”, which issued May 20, 2003, describes a method by which the material of the top electrode is oxidized, thereby changing its acoustic properties. Only the upper portion of the electrode is altered, leaving the remainder of the electrode to conduct current.
Still other methods contemplate patterning part of the resonator using one or more lithography steps to selectively remove material from the acoustic path and alter the resonant frequency. One example of this approach is described in U.S. Pat. No. 6,657,363 to Aigner, entitled “Thin Film Piezoelectric Resonator”, issued on Nov. 8, 2000. In this approach, material is selectively removed from a mass load layer deposited on the electrode to alter the resonant frequency.
Because the resonant frequency of BAW resonators depends on the thickness of the layers making up the device, and because available semiconductor manufacturing equipment deposits materials with both cross wafer and wafer-to-wafer variations in the thickness of every layer, it is not practical to batch manufacture BAW resonators and achieve high yield when the accuracy required of resonant frequency is significantly smaller than 1% (i.e., 10,000 ppm of the frequency).
Drawbacks of these prior art methods can be categorized by increased manufacturing cost and complexity, and the limited accuracy achieved.
Methods that add extra mass to the top of the resonator are limited in the number and range of different resonant frequencies that they can span by the set of binary combinations of the number N of mass load layers in their manufacturing process (2N). Placing multiple mass load layers on the resonator adds significant cost and complexity to the manufacturing process. In addition, when multiple mass load layers are used there must be an etch process that can remove one mass load layer and stop with a good selectivity on the prior mass load layer, or the mass load layer must consist of a bilayer.
Methods that remove material from the acoustic path require expensive test equipment to carefully calibrate the amount of material to be removed. Also, such methods can require additional functional elements around the main resonator to achieve the removal of material. This adds to the cost and complexity of the manufacturing process.
Methods that alter the material properties of layers in the acoustic path require expensive processing equipment to carefully control the chosen material property, without causing undesirable changes in the material properties of other layers. Further, changes in the material properties that control the resonant frequency can cause undesirable changes in other metrics of resonator performance, such as the quality (Q) factor.
Finally, known patterned mass load methods involve the removal of material throughout the thickness of the mass load layer, which is often the electrode layer itself. This can cause undesirable changes in the Q factor. Thus, a method for adjusting the resonant frequency of an array of resonators that does not suffer from these deficiencies is sought.